In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material is made from toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. Heat is applied to the toner particles to permanently affix the powder image to the copy sheet.
High speed commercial printing machines of the foregoing type handle a wide range of differing weight copy sheets. The beam strength of the copy sheet is a function of the weight of the sheet. Heavier weight copy sheets have greater beam strength than lighter weight copy sheets so additional wear may be put on components in the transfer station. This is called tolerance stack-up. At the transfer station, the copy sheet adheres to the photoconductive member. In the electrostatic transfer of the toner powder image to the copy sheet, it is necessary for the copy sheet to be in uniform, intimate contact with the toner powder image developed on the photoconductive surface. Failure to do so results in variable transfer efficiency and, in the extreme, areas of low or no transfer resulting in image deletions. Clearly, an image deletion is very undesirable in that useful information and indicia are not reproduced on the copy sheet. Various methods have been used to minimize the incidence of image deletions. Hereinbefore, mechanical devices such as rollers have been used to press the copy sheet against the toner powder image on the photoconductive surface.
In another embodiment, a blade is used as a pressing member, adapted to move from a non-operative position spaced from the copy sheet to an operative position contacting the copy sheet and presses the copy sheet into contact with at least the developed image on the photoconductive surface in the transfer station to substantially eliminate any spaces between the copy sheet and the developed image.
The method of transfer includes the steps of establishing, at the transfer station, a transfer field that is effective to attract the developed image from the photoconductive surface to the copy sheet. A blade is moved from a non-operative position, spaced from the copy sheet, to an operative position, contacting the copy sheet, to press the copy sheet into contact with at least the developed image on the photoconductive surface in the transfer station. This substantially eliminates any spaces between the copy sheet and the developed image.
In some prior art systems, the transfer subsystem utilizes a Transfer Assist Blade (TAB) that applies force against the backside of the media that enter the transfer zone. This force is required to hold the paper against the photoreceptor so proper transfer of the image can be achieved to the front side of the media. The TAB has to be able to activate as the media enters the transfer zone and apply force just inside of the lead edge of the media, normally about 3 mm. The TAB will continue to apply force on the media until about 3 mm before reaching the trail edge. The TAB must then deactivate away from the photoreceptor belt in order not to swipe through the toner laden control patches within the interdocument zone. This high-speed activation and deactivation requirement repeats for each subsequent sheet and is timed on the order of milliseconds, otherwise either transfer defects or toner contamination on the backside of the prints result. The drive for this mechanism is generally a stepper motor linked to the TAB assembly. The stepper motor translates between the deactivated position, referred to as the “home” position and the activated position of “home” plus 17 motor steps”. The home position is set in manufacturing by pinning the TAB in its home position which is linked to the stepper motor. The stepper motor has an actuator mounted onto its “D” shaft. To set the home position, an adjustable bracket mounting an optical sensor is positioned such that the actuator that is on the stepper motor just trips the optical sensor. However, because of the large tolerance stack-up due to the multitude of parts affecting the linkage, over time the setting of the sensor to the actuator cannot be achieved. This occurs, even though the optical sensor bracket was designed with the maximum allowable adjustment limited by the space constraint within the transfer deck cavity. The inability to adjust the home position has resulted in problems. First, the manufacturing operator may inadvertently improperly set the optical sensor bracket to its limit. In doing so, often the stepper motor over torques when operating at this stressful home position. When the motor over torques, it will skip steps during subsequent operation causing the TAB assembly extrusion to contact the photoreceptor belt and trip a fault. This scenario costs a great deal of money with high spare rates on the transfer deck. Second, when this problem occurs, a rapid re-centered actuator has to be made (rework tool cost) and part shortage to the line. Re-centering the actuator within the sensor position has been shown to be only a short term fix since this had been done before, only to be done again later when the stack-up condition repeats.